HUMAN EMBRYOLOGY

The human ovum, fertilized high in a fallopian tube, is brushed by the hairlike cilia in the tube toward the uterus, where it becomes implanted, that is, attached to and enclosed by decidual tissue of the uterine lining. Studies of primate embryos indicate that, in humans as well as in apes, cell multiplication begins during the journey of the ovum through the tube. The implanted embryo consists of a hollow sphere, the blastocyst, containing a mass of cells, called the embryonic mass, attached by a stalk to one side of the encircling membrane. In a blastocyst less than two weeks old and measuring 1 mm (0.04 in) in diameter, the microscope reveals the amnion (a sac surrounding the embryo), chorion (a membrane that develops around the amnion and lines the uterine wall), yolk sac, and distinct germ layers.

Developing Embryo's First Month
Thirty hours after conception, the fertilized egg undergoes its first cell division. The embryo, as it is now called, continues to divide as it travels down the fallopian tube. It implants in the uterine lining approximately six days after fertilization, a ball of cells with a disk-shaped embryonic mass. In the second week, the placenta begins to form, nourishing an embryo now composed of the three primary types of tissue: endoderm, ectoderm, and mesoderm. The third week sees the formation of the neural tube, precursor to the central nervous system. Blocks of muscle tissue called somites, from which major organs and glands will arise, form along the embryo’s dorsal surface. Blood vessels and the beginnings of the digestive cavity appear by the end of the week. At the close of the first month, all major organs have begun their development. The eyes are visible, the arms and legs begin to bud, and the four-chambered heart beats for the first time.
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In the third week a closed tube appears in which the brain and spinal cord are to develop. Another tube, folding on itself, is developing into the heart, and at about this stage a portion of the minute yolk sac is enclosed in the body of the embryo to form a part of the embryonic alimentary canal. At the beginning of its fourth week the embryo, now about 4 to 5 mm (about 0.16 to 0.2 in) long, has the rudiments of eyes and ears, and each side of the neck shows four gill clefts. A tail is also present.

Early in the second month the buds of the arms and legs appear. The major internal organs begin to take shape, and in about the sixth week bones and muscles begin to form. By the third month the embryo is recognizable as that of a primate, and is now called a fetus. It has a definite face, with the mouth and nostrils distinct, and the external ears are forming. By the end of the eighth week the tail has usually been incorporated in the body, and in the 11th or 12th week the external genitals become evident. The human embryo is especially vulnerable to the damaging effects of X rays, of disease viruses such as measles, and of certain drugs during the fourth to the eighth week of gestation. These agents can result in the death of the embryo or in the birth of a child with deformed limbs or other abnormalities. By the fourth month an embryo has developed obvious human features. For development in the fetal stage, see Fetus. For abnormalities due to anomalous development, see Birth Defects. See also Development; Multiple Birth; Obstetrics.

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